Looks reasonable. As long as you control the A records for NS1.OLDDOMAIN.COM and NS2.OLDDOMAIN.COM it should be possible and it should work. Caveat: obviously this won't work if your clients have hardcoded the old IP addresses anywhere. Make sure there are no more requests to the old servers before turning them off.
An alternative approach would be to keep your old nameservers running but change their configuration to forward everything to the new nameservers.